1. Field of the Invention
The present invention relates to a communication system including a plurality of audio/video devices, and in particular to an audio/video device for connecting to the communication system.
2. Description of the Related Art
Prior art internal communication systems suitable for use, for example, in an automobile include a plurality of audio/video devices (units) which include one or more audio signal sources (such as AM/FM tuners, cassette players and compact disk (CD) players) and/or image signal sources (such as television systems and navigation devices), a control terminal (commander) provided as a user interface and a central control unit interconnected through bus lines (communication lines) and executing communication between the units by sending command signals on the bus lines, each command signal including a transmission source address, a destination address and command information.
FIGS. 20A and 20B show prior art communication system which includes a command input terminal (commander) 1, an analog cassette player 2, an FM/AM tuner 3 and a CD changer 4 each connected to a bus line. The input terminal (commander) 1, the analog cassette player 2 and the FM/AM tuner 3 constitute a head unit (HU) 5. The head unit 5 is a minimum basic unit of a car audio system. In addition, as shown in FIG. 20A, the system includes an optional CD changer 4 connected to the bus.
In this system, an intrinsic logic address is allocated to each of the respective units. Information is transmitted to the units via the bus in discrete signal bursts, or frames, each frame including a transmission source address, a destination address and command information. When the destination address of a unit coincides with the destination address of a transmitted frame sent thereto, the unit accepts the frame and processes the frame.. For example, when an advance command is entered by a user by activating an appropriate key commanded by the key operation of the commander 1 while the CD changer 4 is being played, the commander 1 creates a frame including a transmission source address set to the address thereof, a destination address set to the address of the CD changer, and command information associating with advancing to a subsequent music selection, and then transmits the frame on the bus line. The CD changer 4 receives the frame and advances from a current music selection to the subsequent music selection and then reproduces the subsequent music selection.
Often, the above-mentioned system is upgraded by its owner to include an additional audio signal source which is of the same type as one of the originally-installed audio signal sources, but provides performance qualities which are superior to that of the original audio signal source. For example, when an FM/AM tuner used in a system has only six preset keys for presetting AM and FM channels, and a user finds this unsatisfactory, the user can connect an FM/AM tuner having a greater number of preset keys and superior performance qualities to the system. Thereby, the system includes a plurality of audio signal sources of the same type.
In an audio/video system having respective units interconnected through a bus line, each basic type of unit is assigned predetermined addresses. For example, an AM/FM tuner has an address of 20, a TV tuner has an address of 22, and an analog cassette has an address of 28.
When a system is upgraded to include a unit which is of the same type as an existing unit, a conflict arises because there are two units having the same address in the system. FIG. 20B shows a case in which an audio system is upgraded to include an FM/AM tuner 3' of better quality. Thus, there are the two FM/AM tuners 3, 3' having the same address in the system. In this case, when the commander 1 issues, for example, a channel seek command to the FM/AM tuner, both of the FM/AM tuners 3, 3' simultaneously receive the command and execute the seek operation.
When a seek operation is to be executed, the FM/AM tuner sends a receiving frequency to the commander 1 which is displayed on a display unit. Therefore, each time a receiving frequency changes, the two FM/AM tuners 3, 3' independently send the receiving frequencies to the commander, which results in the following problem. That is, because the number of displayed frequencies is doubled, each displayed frequency alternately appears on the display unit. For example, when one of the FM/AM tuners seeks upwards from 80.0 MHz and the other FM/AM tuner 3' seeks upwards from 82.5 MHz, receiving frequency data 80.1, 80.2, . . . and receiving frequency data 82.6, 82.7, . . . are alternately sent from the FM/AM tuner 3 and the FM/AM tuner 3'. As a result, the display of frequency changes at a speed twice the usual speed such as 80.1, 82.6, 80.2, 82.7, . . . so that the frequency display flickers and inconsistent frequencies are displayed.
Further, the audio signal which is transmitted on the audio line to the speakers is determined by the location of the units on the bus. That is, audio signals are transmitted serially on the bus from unit to unit until selected audio signals are connected to speakers through an output amplifier. Each unit receives an audio signal from a preceding unit and passes either the received signal or a generated signal to a subsequent unit. Each unit passes the received signal when the unit is inactive (not selected), and transmits the generated signal (blocks the received signal) when the unit is active (selected). However, when two units having the same address are active, then the audio signal generated by one of the units will be blocked by the other unit, and only audio signals from the other unit will be transmitted to the output amplifier. In this case, when the upgraded FM/AM tuner is accidentally installed such that the original tuner is located on the bus between the upgraded tuner and the amplifier, then the upgraded FM/AM tuner does not perform usefully in the system and the connection thereof must be changed, which requires a bothersome re-assembly of the system.
Although the above-mentioned example describes upgrading a system with an FM/AM tuner, the same problem arises when the system is upgraded with other units, such as an analog cassette tape player.
In view of the above-mentioned problems, the present invention provides a method of prioritizing units having the same address by which, when there are a plurality of units having the same address in a system, only one of the units (the unit having superior performance characteristics) is automatically activated and the other units are automatically deactivated, such that the audio signal from the superior performing unit is transmitted to the output amplifier.